1. Field of the Invention
The present invention relates to a bipolar transistor and, more particularly, to a high-frequency transistor having a cutoff frequency f.sub.T of 10 GHz or more.
2. Description of the Related Art
In conventional bipolar transistors, techniques used to increase the operating speed can be roughly classified into two types. One is a technique used for discrete devices, i.e., a technique associated with a so-called comb transistor. FIG. 1 shows a sectional structure of such a transistor. Referring to FIG. 1, reference numeral 30 denotes an n+-type substrate; and 31, an n--type epitaxial layer. Reference symbol EB denotes an external base region; IB, an internal base region; and E, an emitter region. Reference numeral 32 denotes an oxide film; 33, a nitride film; 34, an Al external base electrode wiring layer; and 35, an Al emitter electrode wiring layer.
The other technique is associated with a self-aligned transistor used for a high-speed bipolar transistor. FIG. 2 shows a sectional structure of a transistor. Referring to FIG. 2, reference numeral 40 denotes an n+-type substrate; 41, an n--type epitaxial layer; and 42, a field insulating film for element isolation. Reference symbol EB denotes an external base region; IB, an internal base region; and E, an emitter region. Reference numeral 43 denotes a first oxide film; 44, a nitride film; 45, an external base extraction electrode; 46, a second oxide film; 47 and 48, oxide films on an emitter opening side wall portion; 49, a polysilicon side wall defining an emitter opening; and 50, an emitter electrode.
In the comb transistor shown in FIG. 1, the operating speed is increased by shrink forming. The shrink forming technique has been advanced in accordance with the progress of photolithographic and processing techniques and is expected to be increasingly used in future.
The demands for high-speed elements exceed the progress of photolithography. In order to realize a cutoff frequency as high as f.sub.T =20 GHz at a mounting level, the distance between the emitter and base openings must be sufficiently decreased. In this case, micropatterning of aluminum wiring layers is required. For example, micropatterning must be performed to realize the following sizes: the width of an aluminum wiring layer, 1,5 .mu.m; an emitter/base aluminum wiring layer interval, 0.5 .mu.m; and the thickness of an aluminum wiring layer, 3.0 .mu.m in consideration of a current capacity. It is practically impossible to process such aluminum wiring layers. In addition, since a passivation film formed on aluminum wiring layers cannot be sufficiently buried in grooves, each having a very high aspect ratio, between the aluminum wiring layers, so-called cavities are produced. This leads to a deterioration in reliability.
If a plurality of transistors based on the self-aligning technique shown in FIG. 2 are connected in parallel, the aluminum wiring layer interval can be increased within a controllable range by forming the external base extraction electrode 45 on the peripheral portion of each external base region EB.
The field insulating films 42 for element isolation are, however, formed among the plurality of external base regions EB, and the capacitance between the external base extraction electrode 45 on each field insulating film 42 and the major surface of the semiconductor substrate is large. Therefore, if a large number of transistors formed by self-alignment are connected in parallel, a high-speed operation cannot be performed. The region of each field insulating film 42 must be increased in size by a margin corresponding to an alignment precision deviation of photolithography. This deviation cannot be completely eliminated.
As described above, in a bipolar transistor assembly consisting of a plurality of conventional self-aligned transistors connected in parallel, since the capacitance between each external base extraction electrode and the major surface of the semiconductor substrate is large, if a large number of transistors are connected in parallel, a high-speed operation cannot be performed.